1. Field of the Invention
The present invention relates to an A/D converter converting an analog input voltage to a digital output value, a D/A converter converting a digital input value to an analog output voltage, and a voltage source utilizing the D/A converter.
2. Description of the Related Art
An A/D converter compares an analog input voltage with a reference voltage and converts the analog input voltage to a digital output value based on a ratio of the analog input voltage and the reference voltage. For example, a digital output value Dout of a 10-bit A/D converter is expressed by the following expression (1), where an analog input voltage is Vin and a reference voltage is Vref.Dout=(Vin/Vref)×1024  (1)
A D/A converter converts a digital input value to an analog output voltage according to a reference voltage. A voltage source utilizing the D/A converter outputs the analog output voltage from the D/A converter as an output voltage of the voltage source. For example, in an 8-bit D/A converter, an analog output voltage Vout is expressed by the following expression (2), where a digital input value is Din and a reference voltage is Vref.Vout=(Din/256)×Vref  (2)
As is apparent from the expressions (1), (2), A/D conversion accuracy and D/A conversion accuracy depend on the accuracy of the reference voltage. Therefore, with the aim of improving the A/D conversion accuracy or D/A conversion accuracy a bandgap reference circuit, for example, is used for a reference voltage generating circuit in order to obtain a highly accurate reference voltage. A potential difference of a PN junction of a semiconductor has negative linear dependency on absolute temperature under a constant bias current. A potential difference between two PN junctions biased by current densities different from each other is proportional to the absolute temperature. The bandgap reference circuit utilizes these characteristics to generate a temperature-independent, accurate reference voltage.
Further, Japanese Unexamined Patent Application Publication No. 2000-31823 has disclosed an A/D converter capable of providing a high-accuracy digital output value even when a reference voltage fluctuates. This A/D converter first compares a digital output value obtained by A/D converting a voltage of a thermistor dependent on a power source voltage and temperature, with a digital output value obtained by A/D-converting a voltage of a resistor dependent only on power source voltage according to the reference voltage, thereby calculating a resistance value of the thermistor. Thereafter, the temperature at this moment is found using the calculated resistance value and a temperature characteristic of the thermistor, to find a voltage of a diode having only temperature dependency by use of the found temperature. Then, the reference voltage is estimated by an arithmetic operation using the found voltage and a digital output value obtained by A/D-converting the found voltage according to the reference voltage. A highly accurate digital output value is obtainable by correcting the digital output value from an A/D converting circuit according to the estimated reference voltage.
Further, Japanese Unexamined Patent Application Publication No. 2000-201076 has disclosed an A/D converter that controls, according to an A/D conversion rate, current consumptions of a comparing circuit for an analog input voltage and a reference voltage and of a reference voltage generating circuit, and thus is capable of A/D converting at a required A/D conversion rate with high efficiency without any unnecessary power consumption.
Meanwhile, if the user of an A/D converter knows a value of a reference voltage at the time of A/D conversion, he or she can acquire a digital value representing an analog input voltage accurately irrespective of the accuracy of the reference voltage, by correcting a digital output value from the A/D converter with use of the value of the reference voltage at the A/D conversion. However, a standard value of a reference voltage of the conventional A/D converter is specified but not an actual value of the reference voltage. Therefore, the user of the A/D converter cannot know a value of the reference voltage at the time of the A/D conversion. This makes it necessary for a manufacturer of the A/D converter to trim the reference voltage within a predetermined standard range (a range determined using a standard value as a reference) during the manufacturing process of the A/D converter in order to ensure A/D conversion accuracy. If there is an A/D converter with the reference voltage of a range not trimmed within the standard range, it is regarded as defective, resulting in a lowered manufacturing yield of A/D converters. Such a problem also occurs in D/A converters or voltage sources utilizing the D/A converters.